As the sophistication and complexity of petroleum well drilling has increased, so has the demand for comparable increases in the amount of data that can be received from, and transmitted to, downhole drilling equipment. The demand for real-time data acquisition from measurement while drilling (MWD) and logging while drilling (LWD) equipment, as well as real-time precision control of directional drilling, have created a corresponding need for high bandwidth downhole systems to transfer such data between the downhole equipment and surface control and data acquisition systems.
There are currently a wide variety of downhole telemetry systems that are suitable for use in drilling operations. These include both wireless and wired systems, as well as combinations of the two. Existing wireless systems include acoustic telemetry systems, mud pulse telemetry systems, and electromagnetic telemetry systems. In acoustic telemetry systems, sound oscillations are transmitted through the mud (hydroacoustic oscillations), through the drill string (acoustic-mechanical oscillations), or through the surrounding rock (seismic oscillations). Such acoustic telemetry systems generally require large amounts of energy and are limited to data rates at or below 120 bits per second (bps). Mud pulse telemetry systems use positive and negative pressure pulses within the drilling fluid to transmit data. These systems require strict controls of the injected fluid purity, are generally limited to data rates of no more than 12 bps, and are not suitable for use with foam or aerated drilling fluids.
Electromagnetic telemetry systems include the transmission of electromagnetic signals through the drill string, as well as electromagnetic radiation of a signal through the drilling fluid. Transmission of electromagnetic signals through the drill string is generally limited to no more than 120 bps, has an operational range that may be limited by the geological properties of the surrounding strata, and is not suitable for use offshore or in salty deposits. Data transmission using electromagnetic radiation through the drilling fluid (e.g., using radio frequency (RF) signals or optical signals) generally requires the use of some form of a repeater network along the length of the drill string to compensate for the signal attenuation caused by the scattering and reflection of the transmitted signal. Such systems are frequently characterized by a low signal-to-noise ratio (SNR) at the receiver, and generally provide data rates comparable to those of mud pulse telemetry systems.
Existing wired systems include systems that incorporate a data cable located inside the drill string, and systems that integrate a data cable within each drill pipe segment and transmit the data across each pipe joint. Current wired systems have demonstrated data rates of up to 57,000 bps, and at least one manufacturer has announced a future system which it claims will be capable of data rates up to 1,000,000 bps. Wired systems with data cables running inside the drill string, which include both copper and fiber optic cables, generally require additional equipment and a more complex process for adding drill pipe segments to the drill string during drilling operations. Systems that integrate the cable into each drill pipe segment require pipe segments that are more expensive to manufacture, but generally such pipe segments require little or no modifications to the equipment used to connect drill pipe segments to each other during drilling operations.
As already noted, pipe segments with integrated data cables must somehow transmit data across the joint that connects two pipe segments. This may be done using either wired or wireless communications. Drill pipe segments that use wired connections generally require contacting surfaces between electrical conductors that are relatively free of foreign materials, which can be difficult and time consuming on a drilling rig. Also, a number of systems using drill pipes with integrated cables require at least some degree of alignment between pipe segments in order to establish a proper connection between the electrical conductors of each pipe segment. This increases the complexity of the procedures for connecting drill pipes, thus increasing the amount of time required to add each pipe segment during drilling operations.
Drill pipe segments with integrated cables that transmit data across the pipe joint wirelessly include systems that use magnetic field sensors, inductive coupling, and capacitive coupling. Systems that use magnetic field sensors, such as Hall Effect sensors, are generally limited to operating frequencies at or below 100 kHz. Systems that use inductive coupling currently are generally limited to data rates of no more than 57,000 bps. Systems using capacitive coupling require tight seals and tolerances in order to prevent drilling fluid from leaking into the gap between the pipe segments and disrupting communications. Based on the forgoing, existing downhole telemetry systems currently appear to be limited to proven data rates that are below 1,000,000 bps.